1. Field of Invention
The present invention relates to a preparation process of a complex oxide catalyst and the application of the catalyst to the synthesis of the acrylic acid by using acrolein in the presence of the molecular oxygen.
2. Description of Related Arts
It is well known that the gas phase oxidation of the acrolein in the presence of the molecular oxygen can yield acrylic acid. Especially, the complex oxide catalyst of Mo-V system for the above-mentioned gas phase oxidation has been widely studied and applied to the industries, which can yield acrylic acid efficiently. The catalyst system for the acrolein to yield acrylic acid has been improved, but there are still two more problems. One is the hot spot problem in the catalyst bed; two is the life span problem of the catalyst.
In the industry, in order to increase the yield of the target product, the concentration of the acrolein in the raw gas or the airspeed is increased. However, the tremendous heat produced during the high load oxidation reaction cause the high temperature spot in the catalyst bed. The excessive oxidation caused by the high temperature spot can decrease the selectivity and the yield of the main product. The high temperature can also easily cause the decomposition of the catalyst, which will result in the deactivation of the catalyst. Sometimes, the high temperature can also cause temperature jump. Besides, during the oxidation reaction, the deactivation of the catalyst is inevitable. Therefore, in industry, in order to assure the conversion of the acrolein, the reaction temperature needs to be increased. Lowering the initial temperature of the catalyst and reducing the difference between the hot spot temperature and the reaction temperature can help to increase the temperature range of the catalyst, so as to prolong the life span of the catalyst. Moreover, improving the activity of the catalyst can lower the initial temperature of the catalyst and prolong the life span of the catalyst, assuming that the selectivity of the main product is not lowered.
There have been many techniques to solve the hot spot problem. JP 30688 mixes inert substance with the catalyst at the entrance of the reactor. JP 10802 gradually increases the proportion of the active component of the load in the carrier from the entrance to the exit of the reactor. JP 241209 and CN 1070840 gradually reduce the size of catalyst granules disposed from the entrance to the exit of the reactor. JP 336060 discloses a method of reduce the activity of the catalyst by filling alkali metal at the entrance of the reactor. CN 1672790A discloses a method of reduce the initial activity of the catalyst by adding a kind of volatile inorganic ammonium ion as toxic material into the catalyst. CN 1266106C discloses a method of filling catalyst from low activity to high activity along the flowing of the raw gas in the reactor. The above methods can restrain the hot spot temperature to a certain degree, but because the active substance of the catalyst at the entrance of the reactor is less than that at the exit of the reactor, the catalyst at the entrance is deactivated faster than the catalyst at the exit, which will affect the stabilization of the effectiveness of the catalyst during the long-term reaction. Adding toxic components to reduce the activity of the catalyst can restrain the hot spot temperature, but will decrease the conversion of the acrolein in the early stage of the reaction, which will result in low yield; filling low active catalyst has the same problem. Furthermore, after the toxic components are attached onto the active site of the catalyst, the stability of the catalyst will be compromised after long-term oxidation reaction.
Many articles disclose how to improve the activity and the stability of the catalyst. EP427508, EP235760, JP200055, W027437, JP210991, WO9908788, CN1050779C, CN1697692A and CN1112968C disclose a method of adjusting the components of the catalyst to improve the activity and stability of the catalyst. JP847641 and JP847643 disclose that taking solid superacid having an acid strength H0≦−11.93 as carrier in the complex oxide catalyst can improve the activity and stability of the catalyst. CN1853786 discloses that taking solid superacid having an acid strength H0≦−11.93 as carrier contribute the performance of the gas phase oxidation of the acrolein. CN100345631C discloses a method of adjusting the component distribution of the catalyst from the bulk phase to surface to obtain the catalyst of the acrylic acid. JP25914 discloses a method of adding organic acid during the catalyst preparation process to improve the performance of the catalyst. However, the above-mentioned methods can not completely meet the demand of stable high yield of acrylic acid in a long term.
It is well known that the valence state of the vanadium in the oxide containing vanadium in the catalyst can affect performance of catalyst. As disclosed in CN1177763C, the valence state of V in the catalyst can greatly affect the oxidation reaction of the acrolein. When the 99% of V has a valence state of V4+, the catalyst has good performance. Therefore, adjusting the gas components for roasting catalyst can regulate the activity of the catalyst.
During the preparation of the catalyst in the present invention, C2˜C6 diol or polyol is used as reducing agent that can create active phase during the roasting process to increase the activity of the catalyst and lower the initial temperature of the catalyst. The silicon power of Z component is used to remove the heat produced during the reaction in time, so as to ease the accumulation of the heat and reduce the difference between the hot spot temperature and the salt bath temperature. In the above two measures, the catalyst can increase the conversion rate and the selectivity of main product, and can operate within a wide temperature range without temperature jump, so that the stability of the catalyst is improved. The catalyst is used for the acrolein to produce acrylic acid with high selectivity and high yield, and can be applied to the stack gas recycle and non-recycle.